How can I load balance incoming email across a few servers?

There are a few ways to to this. You could setup 1 mx record and have that load balanced via dns.

.domain.net. 86400 IN MX 10 server1.domain.net.

server1 IN A x.x.x.1

server1 IN A x.x.x.2

server2 IN A x.x.x.3

server2 IN A x.x.x.4

This is how Hotmail.com performs load balancing, except that they have multiple MX's and multiple servers